vm: unify _highWrite and _onAbnormalStop

Both _highWrite and _onAbnormalStop should share the same logic about
the volume extension.

Change-Id: I4fd790986b81402847b06810529841cfd7bae119
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20966
Reviewed-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>